Conventionally, in lower body constructions for motor vehicles, it has been so arranged as disclosed in, for example, Japanese Utility Model Laid-Open Publication No. 10974/1982 (Jikkaisho 57-10974) that a pair of cross members are provided on a floor panel having a tunnel portion formed at a central portion thereof in a widthwise direction of a body such that opposite ends of each of the cross members are attached to the tunnel portion and each of opposite side sills, respectively. However, in the case where the prior art lower body constructions are applied to a body construction of a motor vehicle, for example, a cabin floor of a motor truck, in which the floor panel is attached to upper end portions of the side sills, such a problem arises that since it becomes impossible to attach the outer end of each of the cross members to each of the side sills, the lower body construction of the motor truck has insufficient strength in the widthwise direction of the body.
Meanwhile, a lower body construction provided with a drain device for draining electrodeposition liquid after electrodeposition coating is proposed in, for example, Japanese Utility Model Laid-Open Publication No. 121378/1982 (Jikkaisho 57-121378). However, this known lower body construction has such a disadvantage that the drain device is complicated in structure.